This invention relates to curable unsaturated polyester resin compositions.
Recently, unsaturated polyester sheet molding compounds (SMC) and bulk molding compounds (BMC) are coming to be recognized as excellent plastic materials for automotive exterior body panels and are applied not only to main exterior panels such as engine hoods, roofs and trunk lids but also to exterior parts such as spoilers, air-intakes and rocker panels. This invention relates to improved curable unsaturated polyester resin compositions represented by such SMC and BMC.
It is known, however, that unsaturated polyester resins have a large shrinkage when they are cured. For this reason, molded products obtained from such unsaturated polyester tend to easily crack or warp. Glass fibers used as reinforcement tend to become easily visible and pinholes (porosity) appear to adversely affect the surface appearance and paintability. It is also a problem that they have inferior dimensional stability.
In view of the above, it has been known in order to make up for such shortcomings to mix thermoplastic resins such as polystyrene, polyvinyl acetate, polymethyl methacrylate and block copolymers of a conjugated diene monomer and an aromatic vinyl monomer as an additive to unsaturated polyester resins (29th National SAMPE Symposium, Apr. 3-5, 1984). There is yet to be discovered, however, a thermoplastic resin which is satisfactory from all points of view such as compatibility and dispersibility when mixed with unsaturated polyester resins, ability to reduce shrinkage, surface appearance and paintability. At the present time, therefore, they are being used only for some, and not all, of these required characteristics. Since molding compounds to be applied to automotive exterior body panels, in particular, are required to have surface qualities comparable to those of a steel sheet, unsaturated polyester resin compounds containing known thermoplastic resins are hardly satisfactory.
As an additive to be used with unsaturated polyester resins, Japanese Patent Publication Tokkai 60-99158 has disclosed the use of polystyrene-polyester graft copolymers having a polyester chain in the branch. Examples of the method of forming a polyester chain in the branch disclosed in this reference include condensation polymerization between a dibasic acid and glycol, ring-opening polymerization of polycaprolactone and ring-opening polymerization of acid anhydride and alkylene oxide. The two terminal groups of these polyester chains are hydroxyl or carboxylic acid groups and macromonomers are obtained by reacting one of these terminal groups with a reagent of various types in order to introduce a vinyl group and a graft copolymer is produced by copolymerization of such a macromonomer with another vinyl monomer.
Such a prior art method, however, has the following problems. Firstly, since use is made of a macromonomer with hydroxyl and carboxylic acid groups as terminal groups, gels of an unknown structure are easily produced and this makes it difficult to obtain a graft copolymer of a well controlled structure. Secondly, if a macromonomer is obtained by a method described above, bifunctional macromonomers with vinyl groups introduced into both terminals are inevitably generated as by-products and since it is extremely difficult to remove such bifunctional macromonomers by purification, these bifunctional macromonomers eventually come to participate in the copolymerization reaction. As a result, cross-linking gels are produced significantly.
Graft copolymers obtained by copolymerization of ethylenic unsaturated monomer onto the principal chain of polycaprolactone (U.S. Pat. No. 3,760,034) and graft copolymers obtained by copolymerization of polyaduct (such as polyamide and polyether) or polycondensate (such as polyester) onto the principal chain of polydiene-type elastomer (U.S. Pat. No. 4,670,485) have also been considered. According to these references, graft copolymers are obtained by reacting an ethylenic unsaturated monomer or a macromonomer with a polymer on the principal chain of polycaprolactone or polydiene-type elastomer in the presence of an initiator agent for radical polymerization. By these prior art methods, however, the degree of grafting of ethylenic unsaturated monomers and macromonomers is poor and a large amount of homopolymers comes to be mixed with the product. Thus, it becomes difficult to separate the desired graft copolymer from the reaction product. Moreover, it is difficult to control the degree of grafting and the graft chain length, and there also arises the problem that generation of three-dimensional gel materials cannot be avoided.
In summary, such graft copolymers produced by prior art methods cannot possibly satisfy the conditions of high surface flatness and paintability required for the application to automotive exterior body panels.